The present invention relates to a process to obtain molecular orientations in perforated plates made of extruded plastic material and to the product obtained by said process.
It is known that in this field different methods presently exist allowing to obtain, directly from an extrusion step, a perforated plate made of plastic material. For example, one of these methods which has been the subject of the Italian patent application No. 20853A/81 and of the corresponding U.S. Pat. No. 4,470,942 in the name of the same applicant, essentially consists in extruding a plastic material in the form of a plate which, immediately after its coming out of the extruder head, is simultaneously stretched both in a longitudinal and transverse direction. Simultaneously to the stretching action the plate is submitted to the action of punches provided with reciprocating motion in the direction of the plate so as to repeatedly pass through the same in order to make a series of suitably distributed holes on the surface thereof. At the end of the tensioning step, the perforated plate is immediately cooled.
All plates obtained by the process roughly described hereabove or by other known processes are widely used in many fields. For example, they are used to fix boundaries, to accomplish safety barriers, barriers against the wind, the snow, etc.
In the above mentioned uses given by way of example only, the perforated plates made of plastic material have proved to be of low price, long lifetime and high aesthetic quality.
They are however subjected to some drawbacks.
One drawback is given by the fact that in some applications the mechanical resistance of known plates is not quite statisfactory. In greater detail, said plates have a rather reduced tensile strength so that, when they are used for example as safety barriers or barriers against the wind and the snow, they are easily subjected to plastic deformations due to the stresses to which they are submitted. When a perforated plate used as a barrier against the wind or in similar manner undergoes a plastic deformation it is no longer very functional so that it is necessary to replace it.
A further drawback resides in that the above mentioned plates can, under some conditions, undergo dimensional shrinkages, above all if they are exposed to the sun or to rather high temperatures.
It is also to be noted that, owing to what specified above, perforated plates of known type require a particular care at the setting, above all when used as safety barriers or the like. In fact they must be fastened to posts driven into the ground in a suitably spaced relationship, which must be perfectly parallelto each other and at right angles to the longitudinal extension of the respective plates, in order to avoid overtensions or localized deformations taking place on said plates.
In addition these plates can be improved as regards both their weight per surface unit and the amount of material used to produce a plate of given sizes.